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“It is not suffering that diminishes man, but suffering without meaning.”

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Victor Frankl

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Asked to describe their pain, especially chronic pain, patients often appear perplexed, stating, “I don’t know. It just hurts.” Pain is a subject of deceptive complexity. It means different things to different people. Algology is the study of pain, and this chapter discusses a number of important points relevant to its clinical application, pain medicine.

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Before we focus on the patient with chronic pain and a general approach to pain management, basic nosology and terminology need clarification.

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There are a number of ways to classify pain. Some pain specialists separate it into malignant (cancer) and nonmalignant pain. Others divide it into acute, recurrent acute, and chronic pain. Acute pain is short-lived and follows injury or near injury to tissue. Recurrent acute pain is similar in duration but tends to recur. It need not involve injury. Examples are migraine headache and sickle cell vasoocclusive episodes. Depending on the injury, chronic pain is variably defined as that persisting 1 to 6 months after the tissue has healed. One example of chronic pain is postherpetic neuralgia.

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Pain can also be classified in terms of mechanism. Nociceptive pain denotes pain arising from tissue injury, and the degree of pain is usually somewhat proportional to the degree of injury. Nociceptive pain itself may be subcategorized into visceral pain, a dull, crampy, and poorly localizable discomfort—as might be experienced in gastroenteritis—or somatic pain, a sharper and more localizable sensation of the body wall, as might be felt after a laceration. Each type of pain may be mild or intense.

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Neuropathic pain is not nociceptive, and the degree of pain is not proportional to the degree of injury; it is caused by disordered sensory processing of the nervous system and is a pathologic persistence of a normal sensitizing mechanism that can be useful in the setting of acute pain. Neuropathic pain can be subcategorized into central neuropathic pain, which can originate at any level of the central nervous system, and peripheral neuropathic pain, which is generated at the level of a nerve or nerve root. The most famous example of central pain is poststroke thalamic pain; common examples of peripheral neuropathic pain are neuromas, diabetic neuropathy, and complex regional pain syndrome, types 1 and 2 (previously known as reflex sympathetic dystrophy and causalgia). As with central pain, a number of different mechanisms may be involved. Even when the damage occurs in the periphery, such as injury to a nerve, the constant bombardment of sensory neurons in the spinal cord with pain signals from the periphery renders the neurons hypersensitive to all input, even to non-noxious stimuli. Neurons then almost continuously “fire up” the pain pathway. Although this is a normal sequence of events in acute injury (i.e., sensitizing injured areas so that they may be protected from further ...

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